There has been increased interest in recent years in water-dispersible polyester fiber. Such water-dispersible fiber is used in various non-woven applications, including paper-making and wet-laid non-woven fabrics, sometimes as part of a blend, often with large amounts of wood pulp, e.g. for paper-making, and/or with other synthetic fibers, such as fiberglass, but also in applications requiring only polyester fiber, i.e. unblended with other fiber. This use, and the requirements therefor, are entirely different from previous more conventional use as tow or staple (cut fiber) for conversion into textile yarns for eventual use in woven or knitted fabrics, because of the need to disperse this fiber in water instead of to convert the conventional textile fiber into textile yarns, e.g. by processes such as carding, e.g. in the cotton system. It is this requirement for water-dispersibility that distinguishes the field of the invention from previous more conventional polyester staple fiber.
Most such water-dispersible polyester fiber is of poly(ethylene terephthalate), and is prepared in essentially the same general way as conventional textile polyester staple fiber, except that most water-dispersible polyester fiber is not crimped, whereas any polyester staple fiber for use in textile yarns is generally crimped while in the form of tow, before conversion into staple fiber. Thus, water-dispersible polyester fiber has generally been prepared by melt-spinning (i.e. extruding molten polyester) into a bundle of filaments, applying a spin-finish, combining the filaments to form a tow, drawing, applying a suitable coating to impart water-dispersing properties, preferably during the drawing operation, relaxing the drawn filaments at a temperature of 100.degree. to 180.degree. C., thereby preferably curing a preferred water-dispersing coating onto the filaments, and then, generally without any crimping (or with imparting only some mild wavy undulations in some cases so that the final sheet made therefrom has extra bulk and a three-dimensional matrix), converting the tow into cut fiber of appropriately short length. Some prior polyester staple fiber has been prepared in uncrimped form, e.g. for use as flock in pile fabrics, but for such use, water-dispersibility has not been required.
Polyester fibers are naturally hydrophobic, as reported, e.g. by Ludewig in Section 11.1.5 on pages 377-378 of "Polyester Fibres-Chemistry and Technology"--English Translation 1971--John Wiley and Sons, Ltd., which has posed a problem in regard to their suitability for wet-laying processes, as disclosed by Ring et al. in U.S Pat. No. 4,007,083, Hawkins in U.S. Pat. Nos. 4,137,181, 4,179,543 and 4,294,883, and Viscose Suisse in British Patent No. 958,430. These and other references suggest improvements in coatings to increase the ability of polyester fibers to disperse in water, and some new coatings have provided significant improvements, so far as regular polyester fiber is concerned. However, a need still exists for further improvement in water-dispersibility, especially for certain problem types. For instance, it is often desirable to use water-dispersible fiber of low denier, because lowering the denier generally provides better cover, better strength and softer products, but reducing the diameter increases the difficulty (and time) in obtaining a uniform dispersion which can avoid or minimize defects. It would also be desirable to use binder fibers in certain wet-laid products, but this has posed difficulties because we have found that preferred binder fibers have also been more difficult to disperse than regular polyester fibers, which are generally of poly(ethylene terephthalate), whereas preferred binder fibers are copolymers of lower melting point with comonomer residues such as isophthalates, e.g. of about 210.degree. C. or less.
It is therefore an object of the invention to improve the water-dispersibility of polyester fiber, especially such types as may pose special problems. For most of the desired applications, I believe the water-dispersible polyester fiber should also show a low coefficient of friction, both towards other fibers, and towards metals, such as steel.
U.K. Patent No. 1,276,329 (Eastman Kodak) concerns a paper product reinforced with hydrophilic water-dispersible polyester fibers, the surfaces of which have been substantially hydrolyzed. The polyester fiber surfaces are treated with dilute alkali solution to achieve substantial saponification or hydrolysis to improve their dispersibility, so that they can be dispersed without the aid of wetting agents. The polyester tow is preferably drafted in a water bath containing sodium hydroxide (at 68.degree. C. in Example 4) and steamed (at 150.degree. C. in Example 4) to effect the surface treatment. This process has serious processing disadvantages and is believed not to solve the problem. It does not teach the use of any water-dispersing coating.